1. Field of the Invention
This invention relates to a controlled deflection roll or an extended nip press, such as is used in the papermaking industry to process a traveling paper web. More particularly, this invention relates to the piston which is supported in a channel on a support shaft within a controlled deflection roll or extended nip press to bias a shoe outwardly relative to the shaft to engage and control the deflection of the roll shell rotating about the shaft. Still more particularly, this invention relates to a unique seal arrangement between the piston and the side walls of the channel and end dams at the ends of the channel.
2. Description of the Prior Art
In prior controlled deflection rolls, the piston, which actuates a shoe which contacts the inner surface of the rotating roll shell to control the deflection of the roll shell and to load the roll shell into nipping engagement with a mating roll, has a pair of continuous slots extending around the piston in which corresponding seals are disposed to engage the side walls of a channel in which the piston is disposed. At the ends of the piston, the seals contact a flat surface of an end plate which is vertically arrayed in an end dam mounted to the support shaft. The end plate is, in turn, biased toward the seal by hydraulic pressure which is introduced between the end plate and end dam. The end plate is movable relative to the end dam toward the piston and its seal. When the piston is hydraulically actuated to move radially outwardly relative to the support shaft, the same hydraulic fluid is also introduced into a space between the end plate and the end dam which restrains the end plate from movement in any direction except toward and away from the piston in a direction parallel with the longitudinally extending length of the piston in the channel in the support shaft.
The seals in the side slots in the piston are biased outwardly with springs. The end seals are fitted to the ends of the side seals so as to maintain sealing continuity around the ends of the piston. The fluid seal at the ends of the piston is effected, as explained above, by maintaining the end plates disposed in the end dams biased against the end seals by the same pressurized hydraulic fluid which is used to actuate the roll shell-supporting shoe, which is either supported by the piston or is an integral part of the piston. The piston is, thus, sealed about its entire periphery such that the hydraulic fluid which pressurizes and actuates the piston cannot escape around and past the piston to the space in the controlled deflection roll intermediate the rotating roll shell and the support shaft of the controlled deflection roll. The only hydraulic fluid lubricant which is purposely introduced into the space between the roll shell and support shaft is controlled in both quantity and location, such as by introducing the lubricant to the face of the shoe by so-called capillary or throttling tubes extending in the shoe and piston between the channel and pockets in the face of the shoe. This is known as hydrostatic lubrication. Another method of lubricating the interface of the shoe and inner surface of the roll shell is hydrodynamically, such as by introducing lubricant against the inner surface of the roll shell immediately upstream of its contact with the leading edge of the shoe, to provide a hydrodynamic fluid film of lubrication. Both methods are used in contemporary controlled deflection rolls and are contemplated for use in the controlled deflection roll utilizing the seal of this invention, although the method of providing such lubrication does not form part of the invention.
The problem with the prior art seal construction described above is that the position of the end seal against the end plate changes as the piston is actuated relative to the stationary support shaft. This is particularly true in so-called self-loading types of controlled deflection rolls wherein the piston and shoe move a relatively greater distance, such as about 5-8 cm, in order to both position and load the roll shell into nipping engagement with its mating roll. As the end seal moves from a relatively lower position against the end plate to a relatively higher position, the end plate tends to move slightly in its mounting in the end dam due to the space between the plate and end dam which is filled with pressurized hydraulic fluid to effect the sealing engagement between the end plate and end seal. In other words, when the end seal is near the bottom of the end plate, the hydraulic pressure near the top of the end plate tends to move the top of the end plate toward the piston. When the end seal is near the top of the end plate, the hydraulic pressure tends to move the lower part of the end plate toward the piston. Such relative movement between the end seal and the end plate tends to gall the end plate and otherwise cause excessive wear and scuffing to the end plate. Such excessive wear is exacerbated at the top of the end plates due to their being constrained at the top by the end dam lip which prevents the end plate from coming out of its mounting in the end dam. The more restrained the end plates are near the tops of the end dams, the more unrestrained they are near the bottoms of the end dams, so that retraction of the piston in the channel is either restrained, or causes excessive wear against the lower portions of the end plates, or both.
Such a construction results in either the end dam being relatively tall, with a correspondingly tall end plate, or the translational piston movement relative to the support shaft to be limited, or both conditions are present. Neither of these physical or operating conditions permits the controlled deflection roll to operate under a desirably wide range of operating parameters.
Another problem associated with the prior design of end dam and seal arrangement is that in zone-control types of controlled deflection rolls, wherein successive, end-abutting, longitudinally aligned shoe segments are disposed along the longitudinal length of the controlled deflection roll support shaft in order to provide different roll shell support forces at different locations longitudinally along the length of the controlled deflection roll, the end dam and seal arrangement between such end-abutting shoe segments have to be quite wide, such as about 9-10 cm, in order to provide back-to-back end plates in the sealing arrangement. This reduces and limits the area in which the shoes can support the roll shell relative to the support shaft and also reduces the flexibility of the zone control since the shoe segments must necessarily be spaced further apart in the longitudinal direction along the length of the roll.
In a similar manner, the relative thickness of the end dams at the ends of the support shaft in prior sealing arrangements prevents the shoe from extending closer to the ends of the roll shell, and the floating nature of the end plate in the end dams results in the inability of the piston to accommodate a gap between itself and the end dam. The existence of such a gap is desirable, and often necessary, in order to accommodate relative deflection of the support shaft and shoe relative to the roll shell.